Relocation of microseismicity using reflected waves from single-well, three-component array observations: Application to CO2 injection at the Aneth oil field

摘要

Passive microseismic measurement is an important tool for monitoring underground behavior during CO2 geosequestration; such monitoring is necessary for maintaining maximum long-term safety. It is however difficult to deploy an ideal observation network at reasonable cost for obtaining reliable source locations. In this paper, we show the re-analysis results of microseismicity detected on a single vertical borehole array deployed at the Aneth oil field, where CO2 injection was conducted for enhanced oil recovery operation. The objective of the re-analysis was to obtain improved source depth estimates. We first show possible translation of microseismic source locations from preliminary results based on detailed waveform observations and ray-path modeling using a 7-layer velocity model. Then, a source location method using multiple reflection waves and the relative source location method for many small slave events is proposed, and modified source location results are compared with those obtained by the standard location method. From the modified source depth distribution, we can infer that most of the microseismic events likely occurred as a result of injection salt water disposal and not by CO2 injection at the Aneth field. In general, the proposed analytical technique provides a more reliable understanding of underground changes during CO2 injection by improving the depth accuracy of microseismic activity